A double barrier resonant tunnelling diode (RTD) is a semiconductor device having a sandwich construction, in which a middle section is formed of a thin (nanometers) narrow bandgap semiconductor such as GaAs between wider bandgap semiconductors such as AlGaAs. Because of the bandgap difference, a potential well is formed in the conduction band and/or the valence band. The electron or hole motions in the thin well are confined in the growth region, leading to a set of quantized states, which gives rise to the term "quantum well". Because of the layered nature of the device, the in-plane electron or hole motion is still free and one quantum state can be occupied by many electrons or holes, giving rise to the term "subband". Intersubband transitions are direct optical and radiative transitions between these subband states.
RTDs have been used in high frequency applications; for example, oscillations at frequencies up to 712 GHz, trigger circuits up to 110 GHz and harmonic multiplications up to 320 GHz have been demonstrated. Logic circuit applications have also been proposed, and as well switching time of less than 2 ps has been measured.